A bivy sack is a low-profile, waterproof barrier for your sleeping bag that offers an ultra-lightweight alternative to traditional tents. Originally used by climbers who needed lightweight, compact shelters for multi-day ascents, they are also popular among backcountry minimalists who want to shave weight. They don't offer room to move, but they're easy to pack.

Structured Bivy Sacks

Structured bivy sacks, called bivy shelters, have small suspension systems that expand the head area to provide some room to sit up in. This makes the confined space more comfortable fit.

Most bivy sacks use two different fabrics to balance protection and weight. The bottom fabric is heavier for durability. The upper fabric is usually lighter and features a waterproof/breathable laminate.